This invention relates to a film suitable for use as a coating on a glass substrate. More particularly, this invention is directed to a niobium doped tin oxide coating applied onto a glass substrate to produce a low emissivity (low E) glass.
Coatings on glass are commonly utilized to provide specific energy attenuation and light transmittance properties. Additionally, coatings are designed to reduce reflections from interfaces between individual coating layers and the glass when a plurality of coatings are applied onto a glass substrate. The coated articles are often utilized singularly, or in combination with other coated articles, to form a glazing.
The attributes of the resulting coated glass substrate are dependent upon the specific coatings applied to the glass substrate. The coating compositions and thicknesses impart energy absorption and light transmittance properties within the coated article while also affecting the spectral properties. Desired attributes may be obtainable by adjusting the compositions or thicknesses of the coating layer or layers.
A particular variety of glass product commonly used in the building industry is low E glass. A primary advantage of the low E glass is that it provides superior thermal control properties, i.e. it limits the passing of thermal energy (infrared wavelengths) through the glass, while maintaining high transmission of visible light. Low E glass can be produced through a sputter coating (soft coat) or preferably, through a pyrolytic process, for example chemical vapor deposition. Typically, glass produced through a pyrolytic process yields a coating which is less easily damaged and less likely to deteriorate under exposure to air.
Low E glass has significant uses in building products and other applications such as substrates for solar cells, display panels, heated refrigerated displays and computer screens. The lower the E value of the glass the less thermal energy is transmitted through the glass and therefore the easier it becomes to control the internal temperature of a building equipped with the low E glass. In many applications, the low E glass can also preferably be color neutral. This allows viewing through the glass with a minimal amount of color distortion. Low E glass can be used alone, or in combination with additional panes of tinted glass or reflective glass to obtain different appearances and thermal control properties.
Coatings with sheet resistance value less than about 500 ohms per square are generally considered to be conductive coatings. The emissivity of a coated glass article is directly related to its sheet resistance. By lowering the sheet resistance, or increasing the conductivity, of a glass sheet, the emissivity is reduced.
In theory, a coating of pure tin oxide on a glass substrate would have an extremely high sheet resistance. However, in practice, tin oxide coatings typically have a sheet resistance of about 350-400 ohms per square. This is due, at least in part, to an oxygen deficiency in the tin oxide, rendering it at least slightly conductive. Fluorine is often used as a tin oxide dopant in order to increase the conductivity. A fluorine doped tin oxide coating (SnO2:F) can produce sheet resistances as low as about 16 xcexa9/cm2. When tin oxide is doped with fluorine, the fluorine will substitute for oxygen in the compound. This substitution of fluorine for oxygen is a factor in the lowered sheet resistance, due to their differing electron configurations. Other materials have been also used as dopants in various glass coating applications.
It would be advantageous to use a material as a dopant, alone or in combination with fluorine or other dopants, which results in a coating having a comparable or lowered emissivity for a given thickness, while maintaining or improving the ease and cost of manufacture of the coated glass products, and without impairing the optical qualities of the glass.
In accordance with the present invention, there is provided a film suitable for use as a coating on glass. The film is a niobium doped tin oxide which can be produced by combining a niobium source with conventional tin oxide deposition precursors. The amount of niobium present in the film is variable based on the planned application. The coating of the present invention can also be a tin oxide coating doped with both niobium and other known dopant(s), such as fluorine. The coating of the present invention can thus be used as an alternative to, or in conjunction with, fluorine doped tin oxide coatings for glass substrates, especially for use in low E glass.
The present invention further provides a process of making a coated glass sheet, preferably by a pyrolytic process, for example by chemical vapor deposition (CVD), wherein the coating includes a niobium doped layer of tin oxide, or optionally, a layer of tin oxide dual doped with niobium and fluorine.
The Nb doped coating of the present invention can be used as a single layer on a glass substrate, or in conjunction with other possible embodiments of the present invention, may be used as a layer in a multi-layer coating stack. In possible embodiments of the present invention, Nb can be used as the sole dopant for the SnO2 coating, or in the alternative, Nb can be used in conjunction with other dopants, such as fluorine.
The niobium doped tin oxide can preferably be applied pyrolytically, on-line onto a float glass ribbon, by a process such as chemical vapor deposition which is well known in the art.
It is an object of the present invention to provide a coated glass material having an emissivity comparable to or lower than the emissivity of known coated glass products.
Another object of the present invention is to provide a method of making a coated glass article with a reduced emissivity.
It is a further object of the present invention to provide a conductive film that can be pyrolytically deposited onto a glass substrate.